Microcircuit devices are used in a variety of products, from automobiles to microwaves to personal computers. Designing and fabricating microcircuit devices involves many steps; which has become known as a ‘design flow,’ the particular steps of which are highly dependent on the type of microcircuit, the complexity, the design team, and the microcircuit fabricator or foundry. Several steps are common to all design flows: first a design specification is modeled logically, typically in a hardware design language (HDL). Software and hardware “tools” verify the design at various stages of the design flow by running software simulators and/or hardware emulators, and errors are corrected.
After the logical design is satisfactory, it is converted into design data by synthesis software. The design data, often called a “netlist”, represents the specific electronic devices, such as transistors, resistors, and capacitors, and their interconnections that will achieve the desired logical result. Preliminary estimates of timing may also be made at this stage using an assumed characteristic speed for each device. This “netlist” can also be viewed as corresponding to the level of representation displayed in typical circuit diagrams.
Once the relationships between circuit elements have been established, the design is again transformed, this time into the specific geometric elements that define the shapes that will occur to form the individual elements. Custom layout editors, such as Mentor Graphics' IC Station or Cadence's Virtuoso are commonly used for this task. Automated place and route tools can also be used to define the physical layouts, especially of wires that will be used to interconnect logical elements.
The physical design data represents the patterns that will be written onto the masks used to fabricate the desired microcircuit device, typically by photolithographic processes. Each layer of the integrated circuit has a corresponding layer representation in the physical database, and the geometric shapes described by the data in that layer representation define the relative locations of the circuit elements. For example, the shapes for the layer representation of an implant layer define the regions where doping will occur; the line shapes in the layer representation of an interconnect layer define the locations of the metal wires to connect elements, etc. It is very important that the physical design information accurately embody the design specification and logical design for proper performance. Further, because the physical design data, also called a “layout”, is used to create the photomasks or reticles used in manufacturing, the data must conform to the requirements of the manufacturing facility, or “fab”, that will manufacture the final devices. Each fab specifies its own physical design parameters for compliance with their process, equipment, and techniques.
As the importance of microcircuit devices grow, designers and manufacturers continue to improve these devices. Each year, for example, microcircuit device manufacturers develop new techniques that allow microcircuit devices, such as programmable microprocessors, to be more complex and smaller in size. Microprocessors are now manufactured with over 50 million transistors, each with dimensions of only 90 nm. As the devices continue to become smaller, more of them can become integrated into a single chip. Moreover, many manufacturers are now employing these techniques to manufacture other types of microdevices, such as optical devices, photonic structures, mechanical machines or other micro-electromechanical systems (MEMS) and static storage devices. These other microdevices show promise to be as important as microcircuit devices are currently.
As microdevices become more complex, they also become more difficult to design. A conventional microcircuit device, for example, may have many millions of connections, and each connection may cause the microcircuit to operate incorrectly or even fail if the connection is not properly designated. Not only must the connections be properly designated, but the structure of the connections themselves must be properly manufactured. For example, a microcircuit device may have several different conductive or “wiring” layers connected by plugs of conductive material referred to as a “contacts” or “vias.” Referring now to FIGS. 1A and 1B, these figures illustrate an idealized design for a portion of a microcircuit device 101. According to this idealized design, the microcircuit device 101 includes wires formed in a first conductive layer of material 103 and a second conductive layer of material 105 separated by a nonconductive layer of material 107. The conductive layers 103 and 105 then are connected by a conductive plug of metal or via 109 through the nonconductive layer 107. It should be appreciated that these figures are for illustrative purposes only, and thus may omit some features, such as barrier layers of material or detailed topological features, that might occur in an actual structure, for simplicity and ease of understanding.
Although the via 109 of the idealized design shown in FIG. 1 will provide a suitable connection between the conductive layers 103 and 105, variation in local processing conditions during the manufacture of the device 101 may cause a particular via to be too small to provide a suitable electrical connection. For example, as shown in FIG. 2, the manufactured via 109′ is too small to carry a minimum required current between conductive layers 103 and 105. To address this problem, a manufacturer may modify the design of the microcircuit to include a second or “redundant” via as a backup in case the first via is not properly formed during the manufacturing process. More particularly, instead of a single via 109 forming the only transition between two conductive layers (i.e., a “single-transition” via), the device 101 may include two vias 109A and 109B, as shown in FIG. 3. Thus, if a single via is not manufactured correctly, its redundant via may still form the desired connection. A conventional microcircuit may have 15 million vias, of which 10 million may be originally designed as single-transition vias. Identifying and doubling even 2 million of those vias would therefore provide a significant improvement in the reliability of the microcircuit.
Adding redundant vias reduces the occurrence of via failures, but not all vias can be duplicated. For example, the layout of a circuit may only allow room for a single via between two layers of conductive material. Also, the additional metal required to form a redundant via may change the capacitance of the surrounding circuit. If the timing of that circuit is critical, adding a redundant via may cause more problems than it would solve. Identifying an insufficiently redundant via is purely a geometric operation, but determining whether to “fix” a via by adding a redundant via requires source information relating to the entire microcircuit design. The device manufacturer thus cannot simply double each via, but must instead determine which vias can be doubled without impacting the operation of the microcircuit.
Vias have been described above as one example of a microdevice structure that can be designed for greater reliability, but there are numerous aspects of a microdevice design that can be modified to improve the reliability, performance or cost of the device, or a combination of two or more of these features. For example, “critical area analysis” can often be applied to predict the susceptibility of a grid of wires to be shorted by a defect, and designs can be altered to increase the spacings between wires in these critical areas, reducing the susceptibility to failure. Similarly, like vias, “contacts” that connect a polysilicon structure (e.g., a transistor gate) with a metal layer may also be designed for greater reliability.
Another example can be found in the preparation of the layout data for mask or reticle fabrication. Masks and reticles are typically made using large tools that expose a blank reticle using electron or laser beams. The pattern of exposure is used to write the desired circuit patterns on the mask, which in turn is used to print the actual device structures on the wafers. Most mask writing tools are able to only write certain kinds of polygons, such as rectangles or trapezoids, and only if they are smaller that a machine limited dimension. Larger features, or features that are not basic rectangles or trapezoids (which would be a majority of microcircuit features) must be “fractured” into these smaller, more basic polygons for writing. Often, the length of time it takes to write a mask is in direct proportion to the number of polygons into which a layout has been fractured. Clearly, a more efficient fracturing into a smaller number of polygons can improve the throughput of the mask writing tool considerably. This is especially true for the complex feature shapes created when a layout has been modified by RET software, to compensate for the distortions and optical effects that will occur during photolithographic processing. The design of a microdevice therefore can be modified for improved manufacturability at a number of different levels, from the overall arrangement of components to the specific mask shapes used to form those components.
While microdevice designs can be modified for improved manufacturability, these modifications are not typically available to the microdevice designer during the design process. Instead, these modifications are typically provided by the fab that will manufacture the microdevice after the design has been created. The modifications provided by a fab may depend upon, for example, the manufacturing equipment employed by the fab, the fab's technical expertise and its previous manufacturing experience. Some characteristics of a microdevice design will facilitate the fab to implement these modifications, but other design characteristics may hinder the implementation of these modifications.
It would be desirable, therefore, to allow a microdevice designer to incorporate modifications to improve the manufacturability into the design flow for the microdevice design. Further, it would be desirable to provide the designer with some guidance as to how the original design should be modified to improve its manufacturability at the foundry. That is, it would be desirable to provide a designer with guidance on how to design a microdevice so that modifications to improve the microdevice's manufacturability can be more optimally applied by the fab at the time of the microdevice's manufacture.